At first glance, the small golden mussel doesn't look like much of a threat. When clustered on a pier or the bottom of a buoy, the brownish spheres wouldn't draw much attention, especially as they look like several of California's native mussel species.

But the tiny mollusks, which can reach up to 2 inches in length, are a big concern: The California Department of Fish and Wildlife, California State Parks, and the California Department of Water Resources just announced the invasive species has been found in North America for the first time in the Port of Stockton in the Sacramento-San Joaquin Delta -- and they're already in at least one California reservoir.

In the Port of Stockton, the Department of Water Resources discovered the golden mussels during routine monitoring, according to Tanya Veldhuizen, DWR special projects section manager. Staff first noticed the mussels attached to monitoring equipment, and a closer inspection revealed more mussels clinging to nearby buoys. Later that day, more golden mussels were confirmed at O'Neill Forebay in the San Luis Reservoir near Los Banos. In an ironic twist, they were found attached to equipment to monitor for invasive zebra and quagga mussels.

The discovery means the mussels could impact the state's drinking water facilities, as they have in other areas where they've taken hold, such as Brazil's Pantanal and China's Pearl River. In California, two-thirds of residents rely on water from the San Joaquin Delta, and officials are wary of the disruption these mussels could bring. If golden mussels were to spread widely in California, they could pose an expensive threat to the state's extensive clean water infrastructure, like dams, canals and pipelines.

"Golden mussels can clog infrastructure like water intake screens and pipes," said Steve Gonzalez, a spokesperson for the California Department of Fish and Wildlife, noting that the mollusk's adaptivity could make its potential impact even broader than that of other invasive predecessors like quagga or zebra mussels, both native to Eastern Europe. "CDFW is concerned due to the golden mussel's tolerance to a wider range of environmental conditions," Gonzalez said, though he added that it's possible the state's colder water, especially going into winter, could slow the mussels' spread.

The mussels are more than just a threat to California's drinking water infrastructure, however. They're a "significant immediate threat to the ecological health of the Delta," notes the release, and could potentially become a problem on a continental scale. "Without containment, golden mussels are likely to spread to other freshwater bodies in California, and to other ports and inland waters of North America, and abroad," caution the various agencies.

Known scientifically as Limnoperna fortunei, golden mussels are an invasive species from China and Southeast Asia, first identified outside their native range in South America in the 1990s.

Despite their small size, these mussels have the potential to cause significant ecological damage in California's waterways, according to Adeline Yee of California State Parks. Like other mollusks, they have a well-documented track record of disruptive impacts on local ecosystems. And their arrival in California comes at a time when the state's waterways are already under stress from multiple environmental pressures. That includes aging infrastructure, chemical discharge into waterways, and other invasive species, such as northern pike, mud snails in Lake Tahoe and water hyacinth -- a plant with visually appealing blooms that can rapidly cover water surfaces, blocking sunlight and impacting the aquatic life below. Golden mussels are yet another challenge.

Up to 200,000 golden mussels can be a single colony, and as filter feeders, they can consume large quantities of plankton and other microscopic organisms that form the base of the aquatic food chain. This harms native fish species, including endangered species like the Central California Coast coho salmon. "It alters the balance of food in the water column and waterway," Yee explains, and could disrupt the food needed for many of the fisheries that support recreational fishing throughout California parks. Without plankton, the entire food webs can collapse, drastically altering the biodiversity of affected areas.

Californians statewide could face widespread impacts if the golden mussel invasion isn't quickly contained, as invasive mussels have already led to economic losses across the country. In 2021, the Department of Agriculture allocated more than $5 million specifically to monitor and fight quagga and zebra mussel invasions. Researchers at the Center for Invasive Species Research at UC Riverside think the figure is likely much higher, noting that managing quagga mussels in the U.S.'s Great Lakes alone costs governments, businesses and industry roughly $500 million per year.

For now, efforts are concentrated on preventing the spread of golden mussels to other water bodies. Effective immediately, all vessels leaving San Luis Reservoir State Recreation Area are required to undergo an exit check to ensure no mussels are being carried in bilge water or on motors, and Yee says California State Parks has paused the process of allowing boats to be marked as "inspected safe." Instead, all vessels will undergo inspection on arrival. Additionally, residents are encouraged to report any sightings or photos of suspected golden mussels through the CDFW's online Invasive Species Program.